The present invention relates to a copolymer. More particularly, the present invention relates to a film-forming copolymer having excellent physical properties and the process for forming the copolymer.
It is often necessary to heat and dry a coated film to fully reveal the physical properties of a polymer of conventional technology. Generally, the physical properties, such as a strength and elongation, of the film dried at an elevated temperature are superior to those of the film dried at ambient temperature. A film dried at atmospheric temperature has poorer water resistance and blister resistance, especially after long term exposure under humid conditions than a comparable film dried at an elevated temperature. Furthermore, some conventional polymers show poorer film forming ability at low temperature than a comparable film dried at an elevated temperature and are unusable in certain applications. As used herein, the term xe2x80x9cambient temperaturexe2x80x9d usually means a temperature above 0xc2x0 C. and, more specifically, from 5xc2x0 C. to 35xc2x0 C.
The object of the present invention is to provide a copolymer that can provide a film having excellent physical properties even when coated and dried at a low temperature. The film prepared from the coating compositions containing the copolymer of the present invention have excellent physical properties, such as a strength and elongation, as well as an excellent water resistance, weather resistance and thermal stability.
The inventor of the present invention has found that the above problem can be solved by using a copolymer prepared by a multi-step polymerization process and which contains at least two acrylic copolymer portions each having a specific range of glass transition temperatures.
The present invention relates to a core/shell copolymer, comprising:
(a) an acrylic copolymer portion A having a Tg of from 30xc2x0 C. to 70xc2x0 C., preferably 30xc2x0 C. to 60xc2x0 C.; and
(b) an acrylic copolymer portion B having a Tg of from 5xc2x0 C. to xe2x88x9230xc2x0 C., preferably 5xc2x0 C. to xe2x88x9220xc2x0 C.;
wherein said acrylic copolymer portion A is present at a level of from 5 weight % to 40 weight %, based on a total weight of said acrylic copolymer portions A and B;
wherein said acrylic copolymer portion B is present at a level of from 60 weight % to 95 weight %, based on a total weight of said acrylic copolymer portion A and B;
wherein said acrylic copolymer portions A and B contain units derived from same said hard monomer and same said soft monomer,
wherein the total amount of said units derived from said hard monomer and soft monomer in each of said acrylic copolymer portion is at least 70 weight %, based on the weight of each acrylic copolymer portion; and
wherein said core/shell copolymer is prepared by a multi-step polymerization process.
The present invention further relates to a process for preparing a core/shell copolymer, comprising the steps of:
(1) preparing an acrylic copolymer portion B having a Tg of from 5xc2x0 C. to xe2x88x9230xc2x0 C. from a monomer mixture (b); and
(2) preparing an acrylic copolymer portion A having a Tg of from 30xc2x0 C. to 70xc2x0 C. from a monomer mixture (a) in the presence of said acrylic copolymer portion B;
wherein monomer mixture (a) is present at a level of from 5 to 40 weight % based on a total weight of the monomer mixtures (a) and (b);
wherein monomer mixture (b) is present at a level of from 60 to 95 weight %, based on a total weight of the monomer mixtures (a) and (b);
wherein said monomer mixtures (a) and (b) contain the same hard monomer and the same soft monomer; and
wherein the total amount of said hard monomer and said soft monomer in each mixtures (a) and (b) is at least 70 weight %, based on the weight of the mixture.